This invention relates to a method and device for non-destructive measurements of residual stresses and loading stresses which is based on optical holographic interferometry technique, where the holographic interferometer is divided into a hand-held holographic probe which is being installed on the object that is to be investigated and a holographic camera which may be situated in a protected in-door environment. The hand-held holographic probe allows to measure residual stresses on surfaces of an object with high curvatures, in places where access is difficult, and under many weather conditions by a simple hand-held manual positioning of the holographic probe during the measurements.
Optical holographic interferometry technique is well suited for measuring residual stresses caused by technological processes of welding, forging, soldering etc. as well as stresses in an object during the object""s work load.
These applications are useful for fields such as offshore oil industry, shipping industry, air industry, process industry, and all types of constructions where loading stresses and residual stresses are vital or fatigue may cause a problem.
An example of the state of the art for measuring residual stresses in an object by holographic interferometry is given in the journal: xe2x80x9cWelding Engineeringxe2x80x9d 1983, vol. 12, p. 26-28. The article describes a typical device for measuring residual stresses which elements, including a laser, optical elements of a holographic interferometer, and a registering medium are rigidly connected between each other by a common metallic basis for protection against vibrations. Also, the operation of the device is based on optical holographic interferometry technique. The device should be installed onto an investigation object during measurements.
The principle of the residual stress measurements by means of this device can be described as follows: First, a hologram of the investigation area of the object is recorded and developed on a registering medium. Further, the residual stresses in a point of the investigation area of the object is released by drilling a small and shallow hole in the object. Then the registering medium with the developed holographic image and the investigation area of the object with the drilled out hole are simultaneously illuminated by the reference and object beams respectively. An interferogram of the investigation area of the object is formed as a result of interference of the two light waves scattered by the object under its illumination with an object beam before and after drilling the hole.
In the case of a welded seam, for instance of an aluminum plate, the interference pattern consists of two pairs of mutually perpendicular lobes which indicate directions of the main residual stresses, namely in longitudinal (Qzx) and in transverse (Qyy) direction of the welded seam. From the interferogram one can determine the normal components of the surface displacement at the hole edge (Wx and Wy), which are equal to the respective number of interference fringes observed in the chosen direction multiplied by one half of the wavelength and divided by the sine of the incidence angle of the object beam. The main stresses are determined by using the above values of W, and Wy from simplified theoretical expressions which presume that the depth of the drilled out hole (hs) is less or equal to its radius (rs):                               Q          x                =                                                            W                x                                            W                                  1                  ⁢                  x                                                      ⁡                          [                                                r                  1                                /                                  r                  S                                            ]                                ⁢                      {                          E              /                              E                AL                                      }                                              (        1        )                                          Q          y                =                                                            W                y                                            W                                  2                  ⁢                  x                                                      ⁡                          [                                                r                  1                                /                                  r                  S                                            ]                                ⁢                      {                          E              /                              E                AL                                      }                                              (        1        )            
where W1x, W2x are parameters equal to the normal components of the surface displacement at the hole edge along the X-axis for unity values of stresses applied first in the X-axis direction (when determining W1x) and, then in the Y-axis direction (when determining W2x), and which is obtained from the theoretical dependencies of W1x, W2x on the co-ordinate from the center of the hole for different ratios between rs and hs, under unity stress for the studied material. E, EAL, and r1, are elasticity modules of the studied material and aluminum and the unity radius, respectively.
However, the above mentioned device has essential drawbacks:
1) It is necessary to drill holes in the object that is to be investigated for residual stresses. Thus the method is a destructive test, and is obviously not acceptable for a variety of objects and applications.
2) The device does not allow the evaluation of residual stresses in the real-time scale due to use of silver-halide-based photographic emulsions as the registering media. These requires large development times.
3) The device enables only indoor measurements, c.f. under workshop comfortable conditions.
4) The device enables measurements only on horizontal surfaces with a weak curvature, and it does not allow to perform measurements on inclined and vertical surfaces and on hardly accessible places of the investigation object.
An attempt to eliminate the mentioned drawbacks was made in the device for measuring residual stress, described in U.S. Pat. No. 5,432,595 to Pechersky. This device is similar to the device described above, but the release of the residual stresses is achieved by heating the investigation point by radiating it with the infrared (IR)-pulse.
However, this device does also suffer from considerable drawbacks which can be summarized as follows:
1) Deviation of the energy distribution over the IR-pulse cross-section from a rectangular shape as well as the heat dissipation from the investigation point of the object irradiated with the IR-pulse results in a blurring out of the boundaries of the spot where the release of residual stresses occurs. This excludes the use of expression (1) and (2) for quantitative evaluation of residual stresses from the measurements of normal components of the surface displacement. It also makes it difficult to obtain analytical expressions for subsequent quantitative determinations of residual stresses from the measurements of normal components of the surface displacement, and makes the assignment of the determined residual stress of a particular point of the object difficult.
2) Due to the heating of the investigation point up to the transition temperature into the plastic state where the residual stresses are released, the action of residual stresses localized outside of the heated spot will deform the surface of the object, not only in the vicinity of the heated spot but also within the spot itself. This is an additional confirmation for the above given conclusion that this device does not allow the use of the analytical expressions given in equation (1) and (2), since these assume that the stress release occurs in a spot with sharp boundaries and no deformation within the region with released stresses. Further, the problem of obtaining new analytical expressions for quantitative determinations of residual stresses is very complicated due to the uncertainty in the determination of the boundaries of the region of stress release and the deformation of the region of stress release. This allows one to assume that the considered device can only be used, at best, to reveal residual stresses.
3) New stresses are created by structural changes in the irradiated spot which occurs during heating up to the transition temperature by the IR-pulse. These new stresses together with residual stresses localized outside the region of residual stress release, should deform the irradiated region and its surroundings as well.
Therefore it becomes impossible, from the distribution of normal displacement components outside the irradiated spot, not only to quantitatively determine the residual stresses, but even to determine the directions of the main residual stresses.
Thus, none of the considered drawbacks, also including the fist one has not been overcome in the device described above. Thus, the device cannot be considered as a non-destructive device.
The first mentioned drawback has been overcome in a device for measuring residual stresses, where a xe2x80x9cdislocationxe2x80x9d release of the residual stresses was employed (see applicants corresponding Norwegian application no. 20002601). Let us consider this device and stages of its operation in more detail with reference to FIGS. 1-3. The device includes of an optical device (101) and an electronic device for a xe2x80x9cdislocationxe2x80x9d release of residual stresses (111) with electric current supply electrode (114). The optical device (101) is intended for formation and registration of holograms from an area of the object as well as for formation of interferograms of the above area after releasing the residual stresses. It includes of a coherent light source (102), a holographic interferometer with optical elements (103-104) for formation of a reference (105) and object (106) beam, and a recording medium (107). All components in the optical device (101) are rigidly connected with regard to each other. The optical device also includes a device (108) for positioning and fixation on the object (109). The electronic device for xe2x80x9cdislocationxe2x80x9d release of residual stresses (111) with an electric current supply electrode and clamping device (114), is intended for non-destructive release of residual stresses within a certain area (the investigation area) of an object. The electronic device comprises a generator (110) which is able to deliver high-current rectangular pulses (pulse parameters are within the range: amplitude 1-10 kA, duration 20 xcexcs-20 ms and recurrence frequency 0-100 Hz) and an electric current supply electrode with clamping device (114) connected to the generator. The base of the electric current supply electrode is made as a half-sphere with radius 1.5-5 mm. Both the electric current supply electrode (114) and clamping device are located structurally in the optical device (101).
The method for performing non-destructive determination of residual stresses with this device can, as the methods of prior art, be divided into three stages; registration of a hologram of the investigation area of the object, release of residual stresses in a very small region of the investigation area, and formation of an interferogram from the investigation area containing a region with released residual stresses. The interferogram can be employed to determine the normal components of the displacement of the surface at the boundary of the region with released residual stresses, which in turn can be employed to calculate the released residual stresses by using analytical expressions (1) and (2).
A detailed description of this method for performing non-destructive determinations of residual stresses is given in Norwegian application no. 20002601, and is incorporated here by reference. All we need to know is that when the initial hologram of the investigation area of the object (109) is formed and registered, the electric current supply electrode (114) is raised in its upper position in a distance above the investigation area. This constitutes the first stage (see FIG. 1). Then the electric current supply electrode (114) is lowered until a junction between the investigation point of the object and electrode is established, and a pulse of electric current is sent through this junction in order to perform the xe2x80x9cdislocationxe2x80x9d release of the residual stresses in a small region (0.5-1 mm) of the investigation area of the object. During exposure to the electric pulse, an energy transfer from directionally traveling electrons to the dislocations occurs. This phenomena as well as the magneto-dynamical effect of the percussion compression of the investigation area (in which the electron stream is passing) leads to the directional movement of the dislocations, to the decrease of their concentrations and to the release of residual stresses. The release of residual stresses is thus carried out without causing a transition of the material into a plastic state, and it can be done in a region with a sharp boundary. This constitutes the second stage (see FIG. 2). Finally, the current supply electrode is raised to its upper position, and a interferogram from the investigation area of the object is formed in the third stage (see FIG. 3).
The residual stress measurements by the above given device was checked under determination of the normal components of the surface displacement at the edge of the region of dislocation release of residual stresses in a welded seam of flat aluminum plates and subsequent usage of these components for calculations by expressions (1) and (2). These results were compared with measurements on the same weld obtained by the device using hole drilling for residual stress release. Measured residual stresses differed by not larger than 20%.
Thus, the described device allows to perform non-destructive measurements of residual stresses using therewith the world-widely collected experience in calculating the residual stresses by employing the analytical expressions given in equations (1) and (2), as well as results on experimental determination of normal components of the surface displacement at the boundary of the region of stress release.
There are however still considerable drawbacks which limit a wide use of this technique:
1. When installed on the investigation object the device occupies considerable space of the surface of the investigation object comparing to the region of stress release and requires the creation of special multipurpose clamping devices for its installation on the investigation object; this limits the application of the device for inclined and vertical surfaces and makes its usage impossible for curved surfaces and for a variety of hardly accessible areas of the object which generally need to be investigated.
2. The necessity of fixing the device on inclined and vertical surfaces reduce the efficiency of the device operation.
3. The device does not allow performing measurements of residual stresses under arbitrary weather conditions due to peculiarities of operation of the registering media based on amorphous molecular semiconductor (AMS) films which require comfortable conditions for charging of AMS film surface with corona discharge and for development of holograms (see, for instance, the Norwegian patent application 20002948).
The main object of the invention is to provide a device for non-destructive real-time measurement of residual stresses in materials by optical holographic interferometry technique which overcomes the above mentioned drawbacks.
The object of invention is also to provide a device for non-destructive real-time measurement of residual stresses in materials by optical holographic interferometry technique which enables to perform the measurements of residual stresses on inclined and vertical surfaces of the object and for practically all variety of hardly accessible places of objects without requiring any fastening devices.
The object of invention is also to provide a device for non-destructive real-time measurement of residual stresses in materials by optical holographic interferometry technique which enables to perform the measurements of residual stresses under any weather conditions.